In a manufacturing process of a semiconductor device, a photoresist pattern is formed on a semiconductor wafer, which is a substrate to be processed, through a photolithography process to be used as a mask in etching of the semiconductor wafer.
With a recent trend of miniaturization of semiconductor devices, etching also requires microprocessing. To keep up with such trend of micro-etching, the thickness of a photoresist film used as a mask is getting thinner, and the kind of the photoresist is shifting from a KrF photoresist (i.e., a photoresist exposed to a laser beam of which an emission source is a KrF gas) to an ArF photoresist (i.e., a photoresist exposed to a laser beam having a shorter wavelength of which an emission source is an ArF gas) adequate for forming a pattern opening no greater than about 0.13 μm.
Since, however, the ArF photoresist has a low plasma resistance, it suffers a surface roughening during an etching, which hardly occurs when using the KrF photoresist. Accordingly, there occur such problems of a formation of longitudinal strips (striation) on inner wall surfaces of openings, enlargements of openings (i.e., an increase of critical dimension (CD)), and the like. As a result, due to the thin thickness of the ArF resist together with the above problems, it is difficult to form etching holes with a sufficient etching selectivity.
In order to solve the above problems, Japanese Patent Laid-open Application No. 2006-41486 (Reference Document) discloses a technique involving the steps of forming an amorphous carbon film as a sacrificial hard mask on a target layer to be etched and forming thereon a patterned photoresist film; etching the amorphous carbon by using the patterned photoresist film as a mask; and then etching the target layer with a typically used CF-based gas by using at least the amorphous carbon film as an etching mask. By this technique, problems concerning the etching selectivity and the etching shapes can be solved to some extent.
However, in the etching of, e.g., a DRAM capacitor, it is required to form, on an oxide film, holes each having a very high aspect ratio with a width of about 80 nm and a depth of about 2 μm. Further, in next generation capacitors, the width of the hole is required to be reduced less and less to, e.g., about 68 nm and further to, e.g., about 58 nm. With the technique of the Reference Document, however, it is difficult to form holes of such sizes in satisfactory shapes with a sufficient etching selectivity without suffering a bowing phenomenon and the like.